This invention relates in general to compressors and in particular to a new and useful method of controlling compressors in which signals for the operation of a controller for the compressor are filtered.
The invention relates particularly to a method of filtering both a noisy or fluctuating input signal for a controller, in particular a pumping limit controller, for turbo compressors. In such controllers, which are driven by measured variables to which "noise signals" are superposed, filtering the measured variables is extremely problematical because of the time delay each filter brings with it.
Generally, pumping limit controllers are so designed that the permissible minimum throughput or volumetric rate of flow is determined from the ultimate compressor pressure or from the compression ratio and then compared with the compressor throughput. At too little throughput the so-called "pumping" (surge) would occur, where the pumping medium flows back from the pressure side to the suction side in surges or periodically. In the characteristic diagram of a compressor, the pumping or surge limit line separates the stable from the instable area in which pumping could occur. The pumping limit controller sees to it that a blowoff or reorientation valve is opened at the compressor outlet when the actual working point approaches the pumping limit or a blowoff line running parallel thereto. The variable which can change the quickest is the compressor throughput. But the throughput measuring signals are always superposed by a noise signal or by fluctuations, regardless of whether the throughput or volumetric flow rate information is acquired by means of orifice, an aperture, a pressure tube, the pressure loss at the compressor inlet or in another way.
The noise signal has its origin in that eddies will always form at the flow measuring points. The consequence thereof is that the measuring signal, even at constant throughput (constant volumetric rate of flow), is subject to constant fluctuations which are disadvantageous for the succeeding processing circuits such as controls, recorders, displays, data loggers, analyzing instruments, etc.
In such processing circuits the noise is greatly disturbing because it renders the determination of the exact throughput more difficult, as these instruments always pick up a "band width". It is general practice, threfore, to filter such a noise signal out by inserting attenuators or filters so that a useful signal free of noise is generated.
However, the application of such filters in conjunction with pumping limit controllers is very disadvantageous. For, each attenuator or filter causes a delay of the measuring signal, i.e. an actual change in throughput is only transmitted with delay to the succeeding processing circuit.
But since pumping limit controllers are safety controls they must react as quickly as possible to achieve good protection of the compressor against operation in the instable area, i.e. against pumping or surge.